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Lightweight Shock

Lightweight Hammershock Testing

Lightweight Shock Machine (LWSM) Testing per MIL-DTL-901E is the military specification covering shock test requirements for shipboard machinery, equipment, systems and structures. Anyone who produces an item for military use in any of these shipboard categories will likely need to complete this testing. The purpose of this testing is to verify the ability of shipboard installations to withstand shock loadings which may be incurred during wartime service due to the effects of conventional or nuclear weapons. These requirements grew out of the Navy’s experiences during World War II and the subsequent underwater explosion (UNDEX) experiments. In many cases, it was found that critical ship systems could be disabled by the shock effects of nearby weapons explosions that were not a direct hit on the ship itself.

The current “D” revision of MIL-S-901 was released on 17 March 1989 and has subsequently had clarifications issued in the form of a letter from Naval Sea Systems Command (NAVSEA). The most recent letter was dated 21 November 2012 and was titled “Cost Avoidance and Clarification of MIL-S-901D Shock Test Requirements for Shock Qualification of Equipment on Lightweight Shock Machine”. A similar letter was issued covering testing on the medium weight shock machine. The information that follows is based on the aforementioned documents. The MIL-S-901 specification defines not only the test methods, but also the equipment required to perform the test. The equipment defined by this specification consists of three machines (or platforms) that cover the categories of test based on equipment weight (see Categories below). Since the majority of tests for small equipment are carried out on the Lightweight Shock Machine (LWSM), the following considerations are tailored toward the use of the LWSM equipment.

When required to perform a test per MIL-S-901, the equipment for test must be classified according to four criteria as outlined in the specification. These are Category, Grade, Class and Type.

There are three categories (criteria 1 above) of tests based on the size and weight of the equipment to be tested:

Lightweight – Testing performed on the LWSM. Weight of the test item including fixturing as attached to the machine shall be less than 550 lbs. This usually means that the equipment itself and any associated fixture should weigh less than 300 lbs. There are also restrictions on maximum size as well as on resilient mount deflection limits. These will be discussed in more detail later.

Medium Weight – Testing performed on the Medium Weight Shock Machine (MWSM). MWSM tests Simulate hull level inputs and the weight of the test item including fixturing shall be less than 7,400 lbs. Ultimate size and resilient mount deflection limits also apply.

Heavy weight – Testing performed on a standard or large Floating Shock Platform (FSP). FSP tests simulate hull or deck inputs and the weight of the test item is usually > 4,500lbs. This test is used for both shell and wetted surface mounted items and requires a specific test procedure written and approved by NAVSEA in advance of the test. NTS performs MIL-S-901 heavyweight shock testing at our Rustburg, VA facility.

Equipment is classified according to one of the following grades according to its intended use:

Grade A – Items that are essential to the safety and continued combat capability of the ship.

Grade B – Items whose operation is not essential to safety or combat capability of the ship, but which could become a hazard to personnel, to Grade A equipment, or to the ship itself as a result of shock exposure.

Equipment is then classified according to one of the following three classes according to its mounting configuration:

Class I – Equipment required to meet the shock requirements without the use of resilient mounts installed between the equipment and ship structures.

Class II – Equipment required to meet the shock requirements with the use of resilient mounts installed between the equipment and ship structures.

Class III – Unless otherwise specified in section 3.1.6.3 of MIL-S-901, this equipment class is defined as that which has shipboard application both with and without the use of resilient mounts and is therefore required to meet both Class I and Class II requirements.

Tests are then classified according to the following types according to equipment level of assembly:

Type A – A test of the principal unit. This is the preferred test type. Principal units are items which are directly supported by the ship structure or by a foundation directly attached to the ship structure. This includes items mounted in piping systems, ducting systems, and similar systems supported by ship structure.

Type B – A test of a subsidiary component. Subsidiary components are items which constitute the major parts of a principal unit. The shock response of these components is significantly affected by that of the associated principal unit and all associated subsidiary components. The shock responses of the associated principal unit and all associated subsidiary components are significantly affected by that of the subsidiary component.

Type C – A test of a subassembly. Subassemblies are items which are a part of a principal unit or a subsidiary component. The shock response of the subassembly is significantly affected by that of the associated principal unit or subsidiary component but the shock response of the principal unit or subsidiary component is not significantly affected by the subassembly.

Once equipment is classified according to Category, Grade, Class, and Type, it must meet a number of other criteria in order to be successfully tested on the LWSM. The equipment must be able to be accommodated by one of the standard fixtures fitted to the LWSM and defined by the MIL-S-901 specification drawings. There are a total of six standard fixtures defined as follows:

For Base Mounted equipment, the smallest mounting platform that comfortably accommodates the test item should be used. For all fixtures, the practical size limitations cannot be exceeded. This means not only that the equipment service mount must be able to be attached to the required fixture mounting surface, but also cannot be mounted too closely to the main attachment bolts that secure the fixture to the LWSM. This would effectively defeat the mode action of the fixture that serves as the shock input source to the Unit Under Test (UUT). Likewise, for many test items the mounting spacers called out in Figure 7 of MIL-S-901 must be used if the item contains large flat surfaces in order avoid reinforcement of the bulkhead plate by the test item thereby defeating its natural response action. Also, items with resilient mounts or other flexible elements that have capability of deflecting 1.5 inches or more cannot be tested on the LWSM, but must be subjected to Medium Weight or possibly Heavyweight shock testing. Finally, the operating modes or positions of the test item must be considered in order to determine the number of orientations that the item must be tested in. Some clarification for this aspect of the testing was provided in the November 2012 letter from NAVSEA. These are some of the aspects to consider for testing on the LWSM. Of course, there is much more detail in total that is contained in the MIL-S-901 specification and clarification letters. Most test labs that conduct LWSM shock testing will be glad to provide support in determining the total test requirement. As always, the NAVSEA authority has final authority over the conduct of MIL-S-901 tests and is also a good source of information for anyone required to conduct these tests.